The present invention relates to an alternating-current electric motor drive, designed especially for an elevator.
FIG. 1 visualizes a prior-art electric motor drive for an elevator, which is applicable for the control and regulation of an alternating-current motor, preferably a three-phase motor 1. The electric motor drive comprises a frequency converter 2 containing a rectifier 21 and an inverter 22, which are connected together via an intermediate circuit 23. The input of the rectifier 21 is connected to an alternating-current power source, such as a three-phase power source, such as an electric network SV, and its output produces a direct voltage UC for the intermediate circuit, this voltage having either an adjustable or a constant magnitude. The intermediate circuit comprises a capacitor 23a with a large value, generally an electrolytic capacitor, which is connected between the output terminals of the rectifier 21, the direct voltage UC being applied across its terminals. The direct voltage UC is fed to the input of the inverter 22.
The frequency converter 2 also comprises an inverter control unit 24; 24b, by means of which the operation of the inverter 22 and further the electric motor 1 are controlled. If the rectifier 21 is implemented using diode bridges, then its operation is not controlled and it needs no separate control unit. If the rectifier 21 contains controllable semiconductor switches, then the rectifier 21 needs to be provided with a control unit 24; 24a, which is used to control the operation of the rectifier 21 and especially the magnitude of the intermediate-circuit direct voltage UC. By means of the inverter 22, controlled by the control unit 24; 24b, a three-phase voltage and current of adjustable frequency is produced for the electric motor 3 so that the rotational speed of the motor can be varied between zero and a predetermined maximum speed. The inverter 22 is also used to choose the phase sequence, i.e. the direction of rotation of the motor.
Both the rectifier 21 and the inverter 22 are generally implemented using semiconductor switches. Such semiconductor switches include e.g. the thyristor, triac, MOSFET and IGBT (Insulated-gate Bipolar Transistor). The rectifier and the inverter are preferably implemented as identical IGBT (Insulated-gate Bipolar Transistor) bridge circuits. FIG. 2 visualizes such a bridge circuit. The bridge circuit comprises six IGB transistors 3; 31, 32, 33, 34, 35, 36, FIG. 3, connected pairwise in parallel. When the IGBT bridge circuit is arranged to function as a rectifier 21, each supply voltage phase R, S, T is connected to the rectifier input IN1 between the transistors of a corresponding IGB transistor pair 31, 32; 33, 34; 35, 36 and over these to the positive and negative terminals of the output OUT1 of the rectifier. When the IGBT bridge circuit is arranged to function as an inverter 22, its input and output terminals are connected the other way round as IN2, OUT2 than in the rectifier configuration; the intermediate-circuit direct-current voltage UC is applied across the IGB transistor pairs connected in parallel to input IN2, and each alternating-current phase voltage at the output OUT2 is obtained from a point between the transistors of the respective transistor pair.
Provided on the input and output sides, i.e. on the power source side and the motor side, respectively, of the frequency converter 2 are first and second contactors 4, 5. Both contactors have a number of switches corresponding to the number of phases, in this case three switches. Via the first or input contactors 4, the frequency converter 2 is connected to different phases R, S, T of the alternating-current power source, preferably an electric network SV. Similarly, via the second contactors 5, the electric motor 1 is connected to the output terminals of the frequency converter 2. When the contactors 4, 5 are open, no electric power is transferred from the power source via the frequency converter 2 to the motor 1. When the contactors 4, 5 are closed, electricity is supplied from the power source SV to the motor 1. The first and the second contactors 4, 5 are preferably controlled by means of a common control switch 15. The control switch 15 is closed when the contactors 4, 5 are to be closed to start the supply of power to the motor (and similarly the contactors 4, 5 are opened when supply of power to the motor 1 is to be interrupted). The main purpose of the switch arrangement described above, especially in an elevator application, is to guarantee safe operation of the electric motor and the elevator as a whole.
A problem with the use of contactors 4, 5 is that they require regular maintenance and that they have a limited service life (e.g. 2 years). An additional drawback is the noise produced by the operation of the contactors, which is generated by the fast movement and sudden stopping of the connection elements of the contactor. Further problems result from electromagnetic disturbances of the contactors. The contactors also increase the price of the electric motor drive.
Previously known from European patent application EP-1037354 is an electric motor drive for an elevator, which in principle corresponds to the motor drive presented in FIG. 1, with the following differences. No contactors are provided between the inverter and the electric motor. The control pulses to each semiconductor switch IGBT of the inverter bridge circuit are supplied via an optoswitch in which the voltage supply to its light emitting diode is additionally monitored and controlled via a safety relay. When the safety relay is actuated, the supply of voltage to the light emitting diode is interrupted and the control pulses sent by the control unit cannot pass through the optoswitch to the gate G of the semiconductor switch IGBT. The supply of power to the motor via the inverter is interrupted and the motor stops.
A drawback with the electric motor drive according to the aforesaid EP application is that it only provides a partial solution to the contactor problems described above. Furthermore, the inverter input is provided with contactors over which alternating-current electricity is supplied from a three-phase network to the frequency converter and further to the electric motor.